Metabolism — Carbohydrate Digestion

Introduction

Carbohydrate

• Equal proportions of carbon and H20
• C:H:O = 1:2:1

Starch

• Polysaccharide which functions as a carbohydrate store in green plants.
• Range in structure from linear to highly branched.

Dietary Forms

• Polysaccharides
  • Plant sources: Amylopectin, amylose
  • Animal souces: Glycogen
• Disaccharides
  • Lactose and Sucrose
• Monosacchaerides
  • Glucose and Fructose

Absorption and digestion: Takes place throughout GIT

• Conversion of Starches and Glycogen to Di and monosaccharides
  • sucrose: Glucose-Fructose (α-glycosidic)
  • maltose: Glucose-Glucose (α-glycosidic)
  • lactose: Glucose-Galactose (β-glycosidic)
• Initial breakdown in Upper GIT
  • Salivary alpha-amylase in mouth
    • Action terminated by acidic pH of stomach.
• On exiting the stomach
  • Pancreatic alpha-amylase on brush border
    • Maltose by Maltase
    • Maltotriose
    • Glucose Polymers
    • alpha-limit dextrans
  • Lactase
  • Sucrase
• Absorption in intestine (maximal rate = 120g/hr)
  • Galactose (C4)
  • Pentoses (C5)
    • Rapidly absorbed
  • Hexoses (C6)
    • Rapidly absorbed
    • Glucose:
      • Secondary active transport
      • Basal Na/K atpase
      • 20% Cotransport of Glucose/Na from lumen via SGLUT (shares with Galactose)
      • 80% Passive diffusion down concentration gradient.
      • Apically Via Glut2 to portal circulation
    • Fructose (C6)
      • Carrier mediated facilitated diffusion via GLUT5

Transported to the liver via the portal circulation

• 10% stored as glycogen
• 40% converted to TAGs as fat stores
• 50% used in glycolysis (forms ATP for other liver functions)

Gladwin 2016

Answer by Gladwin